Electrochemical system for assessing hybrid coatings for corrosion protection of hot dip galvanized steel in concrete

Abstract

Corrosion of steel in concrete in one of the major causes of structures degradation, requiring expensive rehabilitation works. The using of hot dip galvanized steel (HDGS) has been recognized as one effective measure to increase the service life of reinforced concrete structures in marine environmental. However, the HDGS corrodes in contact with the high alkaline environment of fresh concrete. Although this initial process allows the formation of a protecting layer barrier, the corrosion that occurs immediately after concreting is harmful and chromate conversion layers are usually used to prevent it. Environmental and human health concerns have restrained the use of Cr(VI) and some hybrid coatings have been proposed as alternatives. To evaluate the performance of these coatings, beyond the laboratory film characterization, in situ tests in real conditions should be performed. This paper describes an electrochemical system designed to evaluate the in situ degradation of HDGS coated with different hybrid films embedded in concrete. A two parallel electrodes system was selected, using a stainless steel plate as counter electrode (CE) and a HDGS plate coated with hybrid material as working electrode (WE). The WE to CE ratio of the areas is about 1:1, with a spacing between them of 1 cm. The cells were embedded in a mortar using water/cement = 0.5 and sand/cement = 3. The electrochemical measurements of two produced coatings with different thicknesses was performed during 70 days after concreting through reading the potential difference to the terminals of a resistor of 100 Ω, using an automatic data acquisition system (Datataker DT505). During the first 7 days, the corrosion rates were expected to be higher so was selected a data acquisition period of 1 min and then changed to 5 min. The results of analysis of the WE of the disassembled cells by optical and scanning electronic microscopy (SEM/EDS) shows to be consistent with data obtained by electrochemical tecnhiques. The system revealed to be highly sensitive to the coatings degradation, allowing distinguishing different hybrid coatings with different thicknesses.